A fuel cell is a device that generates electricity by a chemical reaction. Typically, in a fuel cell, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2, is oxidized with the oxygen ions to form water at the anode. Among various types of fuel cells, solid oxide fuel cells (SOFCs) use hard ceramic compounds of metal oxides (e.g., calcium or zirconium oxides) to form components of the fuel cell, such as, for example, the anode, cathode, electrolyte, and interconnect. Generally, the fuel gas is separated from the oxygen gas stream with leak-tight seals. Generally, in SOFCs, leak-tight seals separating the fuel gas from the oxygen gas are exposed to elevated temperatures (e.g. 600-800° C.) during normal operation. Glasses or glass-ceramic materials typically have been used for such leak-tight seals.
Glass materials generally are known to be flexible and compliant at temperatures above their glass transition temperature. Such high flexibility is desirable in order to reduce mechanical stresses caused by the difference in thermal expansion coefficient (CTE) between the sealing material and the ceramic fuel cell component(s) that is being sealed or bonded. Glass materials, however, generally are not chemically stable when exposed to the hydrogen fuel gas or other metallic or ceramic fuel cell components. In addition, glass materials tend to crystallize and lose their flexibility over time when exposed to the elevated temperatures of normal operation of SOFCs.
Although ceramic materials have excellent long term chemical and thermal stability, ceramic materials are generally not compliant at the elevated temperatures of normal operation of SOFCs. Seals and bonds composed of ceramic materials generally induce substantial thermal stresses on the cell components during SOFC operation.
Therefore, there is a need for the development of new sealing and/or bonding materials for fuel cells, in particular materials that can have higher flexibility and also higher mechanical, chemical, and thermo-physical stability.